Expanding the pH Range of Metal−Nucleobase Complexes for Acid−Base Chemistry: Properties of Bis(guanine) Complexes of (bpy)Pt^II with Either Two Major or Major and Minor Tautomers Bonded Simultaneously

Metal nucleobase complexes provide a potential for acid−base chemistry in the physiological pH range and consequently may contribute or be actively involved in catalytic reactions of nucleic acids, notably of RNAs. Expansion of the available pH range is achieved if additional ligands are involved, for example, an aqua ligand or a second nucleobase, and if relevant pK_a values are sufficiently close. Two bpy's (bpy = 2,2′-bipyridine) containing Pt^II complexes have been studied in this context: [Pt(bpy)(9-MeGH-N7)₂](NO₃)₂·H₂O (2) and Pt(bpy)(9-EtG-N7)(9-EtG-N1)·3H₂O (3′) (with 9-MeGH = 9-methylguanine; 9-EtGH = 9-ethylguanine). Relevant pK_a values, as determined by pD-dependent ¹H NMR spectroscopy in D₂O, of the neutral guanine ligands were found to be ca. 7.78 ± 0.01 and 8.38 ± 0.01 for 2, yet 4.00 ± 0.03 and 7.7 ± 0.1 for 3′ (values converted to H₂O) for each of the two guanine ligands. These values suggest that complex 3′ provides a pH range of roughly 4−8 for potential acid−base chemistry, and furthermore that in favorable cases compounds with two ionizable ligands can function as an acid and a base simultaneously. X-ray crystal structures of both 2 and 3′ are presented and, in addition, that of [Pt(bpy)(9-EtGH-N7)₂](NO₃)₂·(9-EtGH)·5H₂O (2′′). Regarding the use of ¹H NMR spectroscopy for the determination of pK_a values, we note that chemical shifts referenced to sodium 3-(trimethylsilyl)propanesulfonate must be treated with caution when applying cationic complexes because of the possibility of ion pairing. It can lead to mistakes in chemical shift values.